The marsh drowns a little more each tide. Not enough to kill it. Just enough to change everything.

Scientists working in the Netherlands have documented something unexpected in the fight against climate change. The very wetlands we're counting on to help us—those salt marshes that store carbon and buffer coastlines—face a cruel dilemma as seas rise. They can excel at one job or the other. But not both.

When Protection Meets Preservation

For decades, coastal engineers pointed to hard structures. Seawalls. Concrete barriers. Dikes built to outlast storms. But as 680 million people worldwide live in flood-vulnerable coastal zones and sea levels continue their relentless climb, a different strategy has emerged. Work with nature, not against it.

Saltmarshes attenuate waves. They trap sediment, building elevation over time. Their roots bind soil, resisting erosion. And they sequester atmospheric carbon at rates that make them among Earth's most carbon-dense ecosystems—averaging 400 metric tons per hectare.

These aren't separate benefits. They're marketed as a package deal: nature-based solutions that simultaneously defend coasts and fight climate change. Install marshes in front of dikes, the thinking goes, and you solve two problems at once.

The research team tested that assumption across a gradient of tidal submersion in the Western Scheldt estuary. What they found complicates the narrative considerably.

A Natural Experiment in Prolonged Flooding

The study location was unusual by design. Perkpolder, a former agricultural area returned to tidal influence in 2015, hosts patches of cordgrass growing at elevations far lower than normal. Where established marshes in the region flood less than 30 percent of the time, these plants endure submersion six to eight hours daily. No tide is ever skipped.

This wasn't marsh decline. The vegetation persisted, protected from wave stress by surrounding seawalls. But it experienced conditions predicted for marshes elsewhere as seas rise faster than sediment can accumulate.

The researchers measured three critical variables. First, carbon decomposition rates, using standardized teabags buried in marsh sediment. Second, sediment stability, testing how easily the soil could be eroded. Third, the biomechanical properties of the cordgrass itself—height, stiffness, breaking strength.

The longer the daily inundation, the clearer the pattern became.

The Carbon Paradox

Decomposition slowed dramatically under extended flooding. Where marshes spent more time submerged, waterlogged sediment became oxygen-starved. Microbial communities that break down organic matter operate inefficiently in such conditions. The result: carbon previously locked in plant material and soil stayed locked.

Stabilization of carbon increased correspondingly. The hydrolysable fraction of organic matter—the portion typically decomposed quickly—converted instead to more recalcitrant forms resistant to breakdown.

This suggests a positive effect. If rising seas create more anaerobic conditions, carbon storage improves. The marsh becomes a more effective sink, pulling CO₂ from the atmosphere and holding it underground.

But the story doesn't end there.

When Defense Crumbles

Sediment stability collapsed under the same conditions. Moisture content rose with flooding duration. Wetter sediment proved mechanically weaker—easier to erode, more vulnerable to scour during storms.

Shear strength, a proxy for erosion resistance, decreased significantly at every depth measured. Penetration resistance followed the same pattern. The marsh platform, critical for reducing wave heights as water moves across its surface, became less stable precisely when storm surge risk increases.

The vegetation showed stress as well. Cordgrass growing at sites with longer inundation emerged shorter and less rigid. Flexural stiffness—the resistance to bending—declined. Breaking points dropped. These plants would snap more easily under wave loading, attenuating less energy as surges propagated inland.

Young's modulus, which accounts for plant material properties independent of stem size, showed no relationship with flooding. The issue wasn't tissue quality. It was growth form. Taller, stiffer stems simply didn't develop under sustained submersion.

The Cascade Effect

Here the trade-off becomes interaction. Weaker sediment doesn't just reduce coastal defense in isolation. It creates conditions for carbon loss through mechanical processes.

When sediment erodes—increasingly likely as stability drops—stored carbon resuspends into the water column. Some redistributes across the marsh. Some exports entirely to open water. Once disturbed, previously preserved organic matter encounters oxygen-rich conditions. Microbial decomposition accelerates. Greenhouse gases release.

The short-term benefit to carbon storage, then, contains the seeds of its own reversal. Lower sediment stability increases erosion probability under extreme events. Over time, as storms breach weakened marsh edges, carbon stocks accumulated over decades or centuries return to the marine carbon cycle.

What appeared initially as a trade-off—better carbon retention, worse coastal protection—evolves into dual failure. The interaction between ecosystem services transforms an initial win into a net loss.

The Variables That Matter

Not all marshes will experience these effects equally. Suspended sediment concentration matters enormously. Where rivers and tides deliver abundant fine particles, vertical accretion can match or exceed sea-level rise. The marsh maintains its elevation relative to mean sea level. Inundation duration stays constant or even decreases.

But in sediment-starved systems—increasingly common where dams trap river-borne material or coastal infrastructure blocks tidal exchange—the marsh platform falls behind. Relative sea level rises even if absolute sea level holds steady.

Landward accommodation space determines whether marshes can migrate inland as seaward edges erode. Where development or hard coastal defense infrastructure blocks migration, marsh extent shrinks. The remaining platform experiences longer, deeper flooding.

Storm frequency and intensity compound these stresses. A marsh coping adequately with mean sea-level rise may fail during surge events, particularly where sediment stability has declined.

Temperature and precipitation changes layer additional complexity. Warming accelerates belowground decomposition in some studies, potentially offsetting the anaerobic preservation effect. Drought stresses vegetation, reducing productivity and thus carbon inputs. Extreme rainfall mobilizes sediment differently.

Implications for Restoration

The findings carry weight for coastal management decisions. Marshes remain valuable for both carbon storage and flood defense. But banking on automatic co-benefits may prove optimistic.

Restoration projects should account for sediment supply explicitly. Where accretion cannot match projected sea-level rise, managers face choices. Accept that coastal defense capacity will degrade over time. Supplement marshes with other interventions. Target restoration to areas where geomorphic conditions support elevation maintenance.

Hybrid approaches—combining natural features with engineered structures—may prove more resilient than purely nature-based solutions in sediment-limited settings. The marsh provides ecosystem services when conditions allow. The structure backstops protection when conditions exceed marsh tolerance.

Monitoring needs to expand beyond simple presence-absence. A marsh that survives sea-level rise but experiences chronic stress delivers different services than a thriving marsh at optimal elevation. Measuring multiple indicators—decomposition rates, sediment properties, vegetation biomechanics—provides earlier warning of degradation.

The Knowledge Gap

Most studies examine individual ecosystem services. Carbon storage gets quantified in isolation. Wave attenuation receives separate analysis. This compartmentalized approach misses interactions that may prove as important as direct effects.

Few experiments manipulate multiple stressors simultaneously. Sea-level rise won't occur alone. It arrives with temperature increase, precipitation changes, storm intensification, and altered biogeochemical cycles. Understanding combined effects requires complex, long-term research programs.

Different marsh types—different vegetation species, different tidal regimes, different latitudes—likely respond differently. The present study focused on one cordgrass species in one European estuary. Generalizing to Gulf Coast marshes, tropical mangroves, or Arctic wetlands demands additional data.

Forward

The marshes at Perkpolder will continue their experiment. Tides will rise and fall. Sediment will settle or erode. Plants will grow or struggle. And researchers will keep measuring.

What they learn may determine how hundreds of millions of coastal residents approach the coming decades. Not whether nature-based solutions have value—they do. But how to deploy them wisely, understanding their limits as clearly as their strengths.

Climate change doesn't offer simple solutions. It offers trade-offs, interactions, cascades of consequence we're only beginning to map. The work of understanding them, though? That we can start now.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1016/j.ecss.2025.109319